In the exponential growth model equation dN/dt = rN

'N' stands for:

Population size

The correct answer is Option (1) – Population size

Exponential growth : Resource (food and space) availability is obviously essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection. Then the population grows in an exponential or geometric fashion.

If in a population of size N, the birth rates (not total number but per capita births) are represented as b and death rates (again, per capita death rates) as d, then the increase or decrease in N during a unit time period t (dN/dt) will be

dN/dt = (b – d) × N

Let (b–d) = r,

then dN/dt = rN

The r in this equation is called the ‘intrinsic rate of natural increase’ and is a very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth. Population size, technically called population density (designated as N).